This invention involves the field of nonlinear optics and is concerned with optical phase conjugation involving mutually incoherent beams of light.
On a fundamental level, nonlinear optics is the study of the intraction of light with matter. This interaction is nonlinear because incident light can change the index of refraction in some materials, thereby affecting the frequency, intensity, and/or phase of the light itself. By providing a means to manipulate these properties of a laser beam, nonlinear optics has made possible new optical applications, including optical information processing, optical computing, laser beam control, and novel optical sensor designs.
The branch of nonlinear optics known as phase-conjugate optics deals with the generation, propagation, and application of phase-conjugated beams of light. If a light beam is considered as the motion of a family of wave fronts in space, the phase-conjugate of that light wave has exactly the same set of wavefronts as the initial wave, but the phase-conjugate wave moves in the opposite direction. Consequently, a phase-conjugate beam can be considered a time-reversed replica of an incident beam, capable of retracing the path of the incident beam. A device generating such a beam is known as a phase-conjugate mirror.
The photorefractive effect is a nonlinear optical phenomenon which occurs in photorefractive crystals, such as barium titanate (BaTiO.sub.3) and strontium barium niobate (SBN), and can be used to achieve phase conjugation. When a photorefractive crystal is illuminated with two mutually coherent laser beams an interference fringe pattern is formed within the crystal. The fringe pattern causes a charge separation, which creates an electric field that, in turn, induces a change in the index of refraction via the Pockel's effect, resulting in a volume index grating that affects the propagation of the light beams in the crystal and allows the exchange of energy between the beams. This energy exchange by means of photorefractive phase conjugation is distinguished by the lack of any phase crosstalk, i.e., one beam can be amplified at the expense of the other without the aberrations and frequency differences of the donor beam being transferred to the acceptor beam. The discovery of this phenomenon has led to a variety of new applications, including beam processing techniques, such as image amplification, laser beam cleanup, and beam combining, as well as device structures such as ring oscillators, laser radars, and sensor protection devices.
The need for two mutually coherent laser beams to achieve photorefractive phase conjugation, however, has limited the usefulness of this technique in some applications. In an optical communications system, for example, it would be desirable for information to be transmitted between widely separated locations on laser beams generated at each location. Phase conjugation could be used, for example, to correct for aberrations caused by atmospheric turbulence. Since multiple lasers generally produce mutually incoherent beams, it would be desirable to be able to efficiently phase conjugate such mutually incoherent beams.